Regulation of the cell surface expression of human BCRP/ABCG2 by the phosphorylation state of Akt in polarized cells

Human breast cancer resistance protein (BCRP/ABCG2) is believed to act as an efflux pump to protect the body from drugs and toxins. BCRP is known to accept many kinds of endogenous and exogenous compounds as substrates and to be localized on the apical membrane of various tissues. Expression of BCRP is also reported on the side population cells, and a recent report suggested involvement of Akt in the modulation of the side population phenotype. In the present study, we have characterized the effect of Akt on the polarized expression of BCRP using LLC-PK1 cells. After treatment with phosphatidylinositol 3-kinase (PI3K) inhibitors, internalization of stably transfected BCRP from the apical surface was observed after immunohistochemical staining, and the relative expression level of BCRP on the cell surface decreased to 49 +/- 14 and 51 +/- 8% of the control for LY294002 [2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one hydrochloride] and wortmannin treatment, respectively. This phenomenon was supported by the observation of internalized BCRP in presence of dominant negative-Akt. When the cells were treated with epidermal growth factor, the cell surface expression of BCRP was increased to 228 +/- 43% of the control accompanied by Akt stimulation. These results suggest that the relative expression of BCRP on the cell surface is regulated by the PI3K-Akt signaling pathway with a positive correlation in polarized cells. Alteration in Akt activities may influence the cellular extrusion of BCRP substrates by modifying epithelial BCRP localization.